This invention relates generally to electroplating methods and apparatus, and more particularly to methods and apparatus for electroplating copper films on semiconductor wafers.
Electroplating is a very old art, dating back to the 19.sup.th century. A simple electroplating apparatus includes a container for an electrolyte and an anode and a cathode immersed in the electrolyte. The power source, e.g. a battery or a power supply, is coupled to the anode and the cathode to cause current to flow through the electrolyte. Part of this current flow is positively charged metal ions which are attracted to and adhere to the cathode or to any conductive material coupled to the cathode. A metal film is therefore developed on a conductive object coupled to the cathode due to the electroplating process.
As noted, electroplating has been used for many years and for a variety of purposes. For example, precious metals such as silver or gold are often electroplated onto less expensive base materials to make jewelry. Electroplating has also been used to develop metal films on semiconductor wafer substrates for a variety of purposes. There is currently a great interest in the production of copper layers or "films" on semiconductor substrates which can be subsequently patterned into high speed interconnect lines.
There are many advantages in using copper (Cu) films for the next generations of semiconductor devices. Currently, aluminum (Al) and aluminum-copper (Al--Cu) alloys are the materials most commonly used to provide electrical connections between devices of an integrated circuit. However, aluminum and aluminum-copper alloys have a relatively high resistivity (compared to copper) which impede high-speed operation of the integrated circuit. That is, as integrated circuits are operated at higher and higher frequencies, the resistivity of the interconnect lines becomes a limiting factor. Copper has a lower resistivity than aluminum or aluminum-copper alloys and, therefore, is becoming increasingly of interest for its use as high-speed interconnect lines.
At the present time, copper is being deposited on semiconductor substrates by three primary processes. These processes are Physical Vapor deposition (PVD), Chemical Vapor Deposition (CVD) and electroplating. As will be noted subsequently, each of these conventional methods has its advantages and disadvantages.
Physical Vapor Deposition is accomplished within large, expensive machines produced by a number of vendors including Applied Materials, Inc., Novellus, Inc., and others. Within these machines, a plasma is developed which creates positively charged ions that are caused to collide with a copper target to produce a shower of copper particles on the surface of a wafer. PVD machines are very expensive, often costing many millions of dollars. In addition, the cost of operation of PVD machines is quite high. While the copper film properties and uniformity of film thickness are typically fairly good with PVD processes, their gap fill and uniformity of gap fill properties are very poor. By "gap fill" it is meant the ability of the process to fill the small gaps between features on the surface on the semiconductor wafer.
Chemical Vapor Deposition apparatus, also made by such companies as Applied Materials, Inc. and Novellus, Inc. are also very expensive machines. In addition, the cost of operating a CVD machine is typically even higher than that of operating a PVD machine. While the film properties produced by the CVD machine are only average as compared to those produced by a PVD machine, the uniformity of film thickness, gap fill, and uniformity of gap fill for a CVD machine are quite good.
The cost of electroplating equipment is quite low compared to that of PVD and CVD equipment. In addition, the cost of operation of electroplating equipment is relatively low. The properties of the films produced by electroplating tends to be quite good, and its gap fill properties are better than that those produced by either PVD or CVD processes. The uniformity of gap fill with electroplating techniques is also the best as compared with PVD and CVD processes. However, a major problem with electroplating techniques of the prior art is a lack of uniformity of the resultant film thickness, as compared to a much better uniformity of film thickness that can be achieved with the PVD or CVD processes.
Since copper has superior diffusion capability through certain other materials and layers of an integrated circuit, and can poison such other materials and layers, a barrier layer is provided over the semiconductor wafer surface prior to the deposition of a copper layer. The barrier layer is universally provided whether a PVD, CVD or electroplating technique is used to produce the copper layer (film). A typical material used for the barrier layer is Tantalum (Ta) although other materials such as Tungsten Nitride (WN), Titanium Nitride (TiN), Tantalum Nitride (TaN), and Tungsten (W) can also be used in the barrier layer.
In addition to a barrier layer, electroplating techniques of the prior art requires a seed layer of copper (a thin starter layer) to be provided over the barrier layer prior to the commencement of the copper electroplating process. This is because the electroplating technique is an electrochemical process which requires a continuous conductive path between an anode and an electrode. As will be discussed in greater detail below, this seed layer requirement of prior art electroplating techniques requires a relatively thick layer of copper film of, for example, 1,000 angstroms in order to provide an even marginally acceptable uniformity of film thickness. This seed layer can be provided by a PVD or CVD process, although this will substantially reduce the quality of the gap fill and the uniformity of the gap fill in the final film.
In FIG. 1, a conventional copper electroplating apparatus 10 includes a container 12 containing an electrolyte 14, such as copper sulfate (CuSO.sub.4). An anode 16 is immersed within the electrolyte 14, and a cathode is partially immersed within the electrolyte. The cathode 18 is connected to the negative (-) terminal of a power supply 20, and the anode 16 is coupled to the positive (+) terminal of the power supply 20. While the power supply 20 is illustrated, in this example, as a battery, it will be appreciated to those skilled in the art that the power supply is more typically a voltage regulated AC-to-DC power supply.
A semiconductor wafer 22 is supported by a bottom surface 24 of the cathode 18. A number of contacts 26 make an electrical connection between the cathode 18 and the seed layer 28 formed on the active surface of the wafer 22.
In FIG. 2, a view taken along line 2--2 of FIG. 1 illustrates three contacts 26. The contacts and the rest of the chuck are insulated from the electrolyte such that only the wafer is exposed to the electrolyte. That is, the chuck and the contacts are preferably constructed primarily from an organic non-conductive material (e.g. a plastic) such as polypropylene or Teflon. The number and positioning of these contacts 26 are for the purpose of example, and it should be noted that fewer or more contacts can be used and that the contacts may be distributed around the perimeter of the wafer. However, the contacts 26 are preferably positioned at the perimeter of the wafer to reduce the amount of unusable area of the wafer. This perimeter position of the contacts results in a radially variable IR drop in the seed layer 28. That is, at the perimeter of the wafer the voltage V is high, and current I is high as indicated by the arrow 30, while in the central areas of the wafer the voltage V is low and the current I is low as indicated by the arrow 32. For this reason, the uniformity of the thickness of the electroplated film is difficult to control and in general quite poor.
The operation of a prior art copper electroplating apparatus 10 will be discussed with reference to both FIGS. 1 and 2. Positively charged copper ions (Cu.sup.+) will be attracted to the cathode 18 and will be repelled by the anode 16 due to their relative negative and positive charges. These copper ions will electrochemically plate onto the seed layer 28 of the wafer 22. It will be appreciated that the copper ions in the electrolyte form a part of the electrical circuit as charge carriers which allows the current to flow between the positive and negative terminals of the power supply 20.
With the foregoing discussion, it is clear why the uniformity of the film thickness for prior electroplating techniques tends to be poor. Since the voltage and current near the perimeter of the wafer tends to be substantially higher than the voltage and current near the central portions of the wafer, copper plates onto the surface of the wafer 22 much more rapidly towards the edges of the wafer. This phenomenon can be reduced providing a thicker seed layer 28. However, as noted previously, the thicker seed layer results in poorer gap fill and uniformity of gap fill. In consequence, it has heretobefore been difficult to electroplate copper films on semiconductor wafers with both good uniformity of thickness and good gap fill properties.